Page 144 - Fluid Mechanics and Thermodynamics of Turbomachinery

Axial-flow Turbines: Two-dimensional Theory 125

FIG. 4.23. Arrangement of Wells turbine and oscillating water column (adapted from
Raghunathan et al. 1995).

efficient and economical means of converting an oscillating flow of energy into
a unidirectional rotary motion for driving an electrical generator. A novel solu-
tion of this problem is the Wells turbine (Wells 1976), a version of the axial-flow
turbine. For countries surrounded by the sea, such as the British Isles and Japan to
mention just two, or with extensive shorelines, wave energy conversion is an attrac-
tive proposition. Energy conversion systems based on the oscillating water column
and the Wells turbine have been installed at several locations (Islay in Scotland
and at Trivandrum in India). Figure 4.23 shows the arrangement of a turbine and
generator together with the oscillating column of sea-water. The cross-sectional area
of the plenum chamber is made very large compared to the flow area of the turbine
so that a substantial air velocity through the turbine is attained.
One version of the Wells turbine consists of a rotor with about eight uncambered
aerofoil section blades set at a stagger angle of ninety degrees (i.e. with their chord
lines lying in the plane of rotation). A schematic diagram of such a Wells turbine
is shown in Figure 4.24. At first sight the arrangement might seem to be a highly
improbable means of energy conversion. However, once the blades have attained
design speed the turbine is capable of producing a time-averaged positive power
output from the cyclically reversing airflow with a fairly high efficiency. According
to Raghunathan et al. (1995) peak efficiencies of 65% have been measured at the
experimental wave power station on Islay. The results obtained from a theoretical
analysis by Gato and Falc` ao (1984) showed that fairly high values of the mean effi-
ciency, of the order 70 80%, may be attained in an oscillating flow “with properly
designed Wells turbines”.

Principle of operation
Figure 4.25(a) shows a blade in motion at the design speed U in a flow with an
upward, absolute axial velocity c 1 . It can be seen that the relative velocity w 1 is
inclined to the chordline of the blade at an angle ˛. According to classical aerofoil